The present invention relates to a thermal joint for transferring heat from a first surface such as a surface of an integrated circuit chip to a second surface such as a surface of a cooling hat. Specifically, the invention relates to a thermal joint comprising a first relatively thick layer of high bulk thermal conductivity material and a second relatively thin layer of lubricant. The thermal joint completely fills the gap between the first and second surfaces while enabling relative sliding motion to compensate for any lateral distortion. The resultant thermal joint exhibits high areal thermal conductivity.
High speed computers and other fast electronic systems often require assemblies of many integrated circuit chips where each chip contains many active devices, and many chips are spaced closely together. During normal operation the devices dissipate a very large power density, especially bipolar transistor devices. Proper electronic operation of the devices necessitates a cool operating temperature which, in turn, requires adequately cooling of the power density. Conversely the maximum allowable operating temperature of the devices and integrated circuit chips in combination with the limited cooling capability presently available limit the allowable power density, circuit density and system speed. Improved device and integrated circuit chip cooling results in increased permissible power density, circuit density and system speed.
The prior art contains many forms of cooling. One form of cooling which has been proposed is the use of a metal plate held against circuit chips by springs, as is disclosed by Cutchaw in U.S. Pat. No. 4,381,032. A heat exchanger incorporating a deflectably movable diaphragm in forced engagement with an integrated circuit package is disclosed by Cutchaw in U.S. Pat. No. 4,341,432. Another form of heat exchanger for cooling electronic circuits provides for passages within which liquid coolant is circulated, the coolant contacting a flexible wall which is urged against the circuitry to be cooled as shown by Wilson et al, in U.S. Pat. No. 4.072,188. Another form of heat exchanger employs coated metallic dendrites which are held by springs against a circuit chip as disclosed by Babuka et al in U.S. Pat. No. 4,254,431. Yet another form of heat exchanger employs a pillow structure formed of a film and filled with a thermal liquid material for extracting heat from an electric circuit, as is disclosed by Spaight, in U.S. Pat. No. 4,092,697. Also a malleable dimpled wafer is deformed by pressure between a heat source such as an electronic circuit and a heat sink, as is disclosed by Rhoades et al in U.S. Pat. No. 4,151,547. Other United States patents showing a single layer of material interposed between a circuit chip and a cooling device are Steidlitz, U.S. Pat. No. 4,069,497; Balderes et al, U.S. Pat. No. 4,233,645; Yoshino et al, U.S. Pat. No. 4,546,409; Kohara et al, U.S. Pat. No. 4,561,011; Hassan et al, U.S. Pat. No. 4,607,277; Watari, U.S. Pat. No. 4,612,601; Ostergren et al, U.S. Pat. No. 4,639,829; and Meagher et al, U.S. Pat. No. 4,462,462. The use of liquid and reentrant cavities at a thermal interface is disclosed by Pease, Tuckerman and Swanson in U.S. Pat. No. 4,567,505. The use of a composite structure of a conformal coating plus liquid at a thermal interface is disclosed by Berndlmaier et al, in U.S. Pat. No. 4,323,914. A theoretical discussion of cooling considerations is presented in an article in the IEEE Electron Devices Letters, "High Performance Heat Sinking For VLSI" by D. B Tuckerman and R. F. W. Pease, Vol. EDL-2, No. 5, May 1981. Broadbent, U.S. Pat. No. 4,602,314 and Sherman, U.S. Pat. No. 4,258,411 disclosed a flexible thermally conductive body disposed between a semiconductor device and a beat sink. U.S. Pat. No. 3,626,252 discloses a silicone grease loaded with thermally conductive particles disposed between a heat sink and an electronic device.
A well known thermal joint is a single thin layer of oil disposed between a chip and a cooling means. A crude thermal joint is a dry joint between a chip and a cooling means. Such a thermal joint provides contact only at tiny asperities, and everywhere else there are tiny air gaps and poor thermal conduction.
The foregoing cooling systems are inadequate for modern electronic systems, particularly bipolar clips packaged closely together in a Multi Chip Module. Therefore, a piston-linkage cooling system has been used. One example of such a cooling system is described in the IBM Journal of Research and Development, Vol 26 No. 1, January 1982. In the described arrangement approximately 100 bipolar semiconductor chips are each bonded face down. Numerous small solder balls connect each chip to a common printed circuit. The solder ball is a Controlled Collapse Chip Connector (so-called C4 connectors), and the printed circuit is a Multi-layer Ceramic substrate. Adjacent to these chips is a cooling hat. Each chip is adjacent to a small piston disposed in a socket in a water cooled metal block. During operation, each chip generates heat which is removed. The heat is conducted from the back of the chip, across a small gas-filled gap to the tip of the piston, along the length of the piston, across another gap to the socket, through the metal block, and finally is removed by convection into the flowing water. In some modifications the piston tip is made flat for better thermal contact with the chip, there is oil between the chip and the piston, and there is a thermal paste between the piston and the socket. The modifications provide a certain degree of improved cooling ability.
The piston is designed for movement within its socket to compensate for manufacturing tolerances and thermal distortions. To compensate for chip height variations, the piston is made to slide in the socket in a direction perpendicular to the chip surface. To compensate for chip tilt variations, the piston is designed to tilt within the socket. To compensate for lateral distortions (due to non-uniform thermal expansion), the piston tip is able to slide laterally over the chip surface or alternatively to slightly rotate or slide laterally in its socket. The various compliance modes prevent chip-to-chip variations from causing large stresses and hence damage. However, achieving the compliance modes requires sufficient clearance between the chip and the piston, and between the piston and the socket. Each clearance adds thermal resistance to the design.
The prior art cooling schemes contain shortcomings and limitations. In order to effectively remove heat from high power density chips, where many chips are closely packaged on a multichip module, a "tight" thermal path is required from each chip to the coolant. The tight thermal path requirement conflicts with a "loose" path requirement such as is required for the above described compliance modes. In order to protect the fragile C4 connectors, the cooling system must not apply large stresses. Unfortunately, manufacturing variations result in significantly different chip heights and tilts. Also, differential thermal expansion significantly distorts the geometry of the system components so that a completely rigid system would develop excessive stresses. Temperature changes cause thermal expansion or contraction of the chips, substrate and cooling hat. The thermal expansion depends on the material involved, and is generally different for each element. During start up and cool down, there is non-uniform temperature and expansion, which also causes unequal thermal expansion and resultant thermal distortions. If not compensated, damaging stresses may develop at interconnections between the elements. For example, unequal thermal distortion parallel to the substrate surface will cause shear stress and eventual failure of the C4 balls. Such a failure mode must be prevented.
As electronic systems continue to advance, the piston linkage and other cooling systems noted above become inadequate. In some cases, there is too much thermal resistance. In still other cases, the cooling does not provide adequate compliance to counteract variations and distortions. In still other cases, the system is excessively complex when applied to a Multi Chip Module containing many chips.
One example of the general problem is the design conflict manifest in piston cooling. To improve heat transfer typically requires tighter clearance, tolerance, and smoothness (from chip to piston tip, and from piston to socket). By contrast, adequate motion and economical manufacturing favor a design with looser clearance, and the like.
One partial solution is to use oil or thermally enhanced paste in the gaps between elements (i.e., between the chip and the piston, or between the piston and the socket). Another partial solution is to design a more sophisticated geometry in which the piston and block are reshaped to increase their contact area. For example, the piston may be reshaped to increase the area adjacent to the metal block. Nevertheless these partial solutions while beneficial do not fully resolve the design conflict.
A single chip module in combination with a printed circuit board may also be employed to package many chips close together. An example is a dual in-line package containing one chip, attached leads, and a plastic housing. The single clap modules are mounted on a common printed circuit board. Some applications might employ single chip modules with a printed circuit board to achieve close chip packing density and high chip power density. Even using such designs, when there is very high performance, a design conflict is manifest between the requirements of a "tight path" for high thermal conductivity and a "loose path" for mechanical compliance of the cooling system element.
Patent applications, entitled "Convection Transfer System" and "Compliant Fluidic Cooling Hat", assigned to the same assignee as the present application, also concerning heat transfer and cooling are being filed concurrently with the present application and are incorporated herein by reference.